1. Field of Invention
The present invention relates to wireless communication devices that participate in wireless data sessions, and more particularly to hand off wireless data sessions from a first entity to a second entity.
2. Description of Related Art
In a cellular wireless data communication system, an area is divided into a plurality of cells, each defined by a radiation pattern from a respective base transceiver station (BTS). Each BTS is in turn coupled with a radio network controller (RNC), and each RNC is then coupled with a gateway such as a packet data serving node (PDSN), which provides connectivity with a packet-switched network such as the Internet. In a typical arrangement, each RNC serves a plurality of BTSs, and each PDSN serves a plurality of RNCs. Further, each cell is typically divided into a plurality of sectors. A wireless communication device may then operate within the coverage area of a given sector and communicate, wirelessly with the BTS and via the RNC and PDSN, with an apparatus on the packet-switched network.
Generally speaking, the RNC functions to manage allocation of radio resources, such as to allocate radio traffic channels for use by wireless communication devices and to manage handoff of wireless communication device communications between coverage areas served by the RNC. Further, the RNC functions as an interface between wireless communication devices and the PDSN, so as to facilitate establishment of packet-data connectivity for wireless communication devices.
Air interface communications between the BTS/RNC and wireless communication devices can comply with various protocols, examples of which include CDMA, TDMA, GSM, and AMPS. This description will focus on CDMA, although it should be understood that the principles described herein can be extended to apply with respect to other air interface protocols as well.
In CDMA, each sector is identified by a unique “PN offset” in a given carrier frequency, and a wireless communication device communicates with its serving BTS on one or more coded traffic channels each identified by a unique “Walsh code.” High speed data communications under the emerging 1XEV-DO standard (IS-856) occur within a common traffic channel (i.e., using a common Walsh code), with each wireless communication device being assigned to communicate on a particular TDM time slot in the channel.
To acquire packet-data connectivity, according to IS-856, a wireless communication device first sends to its RNC a packet-data-connectivity request called a UATI (Universal Access Terminal Identifier) request. This is an IP-based message, which passes over an all IP channel between the wireless communication device and the RNC. (For this purpose, the wireless communication device has a local non-routable IP address usable for communications with the RNC.)
Once the RNC receives the UATI request from the wireless communication device, the RNC authenticates the wireless communication device and facilitates assignment of a mobile-IP address for use by the wireless communication device. In particular, the RNC sends an access request to an Access Network AAA (ANAAA) over an A12/13 interface, and the ANAAA authenticates the wireless communication device. The RNC then assigns radio resources for the data session, by directing the wireless communication device to operate on a particular TDM time slot in the common data traffic channel on a given carrier frequency. Further, the RNC signals to the PDSN, and the PDSN and wireless communication device then negotiate to establish a point-to-point protocol (PPP) data link connection. In addition, the PDSN signals to a mobile-IP home agent (HA), which assigns a mobile-IP address for the wireless communication device to use, and the PDSN passes that IP address via the RNC to the wireless communication device. Given its radio link (TDM time slot/frequency), data link (PPP link with the PDSN) and IP address, the wireless communication device may then begin communicating with an apparatus on the packet-switched network.
As noted above, one of the functions of the RNC is to manage handoffs of wireless communication device communications between coverage areas. In typical practice, handoff is triggered based on an analysis of the strength of pilot signals that a wireless communication device is receiving from its current serving sector and from adjacent sectors. If the strength of the pilot signal from an adjacent sector exceeds the strength of the pilot signal from the current sector by a threshold level, then handoff will occur. In CDMA, a wireless communication device has an “active set” of sectors in which it operates concurrently. Consequently, handoff may occur when the strength of a pilot signal from an adjacent sector (not in the active set) exceeds the lowest strength of a pilot signal in the active set by a defined threshold level, TADD.
Handoff can be mobile-initiated or network-initiated. In a mobile-initiated handoff system, the wireless communication device regularly analyzes its received signal strength to determine when the strength of a pilot signal from an adjacent sector exceeds the strength of the pilot signal from the current sector by the threshold and, if so, notifies the RNC. The RNC may then responsively direct the wireless communication device to hand off to the new sector. In a network-initiated handoff system, on the other hand, the wireless communication device regularly sends measurements of its received signal strength to the RNC, and the RNC determines when the received signal strength of a pilot signal from an adjacent sector exceeds the received signal strength from a current sector by the threshold and, if so, directs the wireless communication device to hand off to the new sector.
This handoff process is largely seamless when the source and target sectors for the handoff are both within the same RNC serving area. However, difficulty arises when the source and target sectors are served by different RNCs. When that happens, the handoff process contemplated by IS-856 involves releasing the wireless communication device's radio link (TDM time slot) with respect to the first RNC and then forcing the wireless communication device to re-acquire a radio link with respect to the second RNC.
More particularly, when the first RNC detects that the wireless communication device should hand off to a sector served by the second RNC, the first RNC will transition the wireless communication device into a “dormant” state in which the wireless communication device will maintain its data link (PPP session) and IP address but in which it will have no radio link. To do so, (i) the first RNC will signal over a forward link control channel to the wireless communication device, to tell the wireless communication device that the wireless communication device's assigned TDM time slot is being released and that the wireless communication device should operate on the target sector's PN offset instead, (ii) the first RNC will release the assigned TDM time slot, and (iii) the first RNC will signal to the PDSN to direct the PDSN to put the wireless communication device into a dormant state, thereby causing the PDSN to begin buffering any packets that arrive for the wireless communication device.
When the wireless communication device receives the message indicating that it has lost its traffic channel, the wireless communication device will automatically seek to re-acquire packet-data connectivity by sending a UATI message in the target sector. As the target sector is served by the second RNC, the UATI message will then pass to the second RNC. In response, the second RNC will then assign a TDM time slot for use by the wireless communication device.
Further, after the second RNC assigns the TDM time slot, the second RNC can send a signal to the PDSN in response to the UATI message. In order for the second RNC to be able to signal the PDSN, the second RNC needs to know the IP address of the PDSN. The second RNC may learn the IP address of the PDSN in various ways. As an example, the wireless communication device may send the IP address of the PDSN to the second RNC via the assigned TDM time slot for the target sector. As another example, the first RNC may send a message that indicates the IP address of the PDSN to the second RNC prior to handoff of the data session. Other examples of how the second RNC learns the IP address of the PDSN are also possible. After learning the IP address of the PDSN, the second RNC, in response to the UATI message, can signal the PDSN to transition the wireless communication device back to an “active” state.
The transitioning of the wireless communication device's data session from active to dormant and then back to active again, to facilitate a handoff from a first RNC to a second RNC, can disrupt data communications and is generally inefficient. Therefore, an improvement is desired.